Braking function for brushless DC motor control

Abstract

A motor control circuit that features a smart, two-phase braking operation is presented. The motor control circuit includes a motor drive circuit to apply a brake current to a coil of an external motor for active braking of the motor. The motor control circuit further includes a braking control circuit, coupled to the motor drive circuit and responsive to an externally generated control signal, to control the active braking by the motor drive circuit so that the active braking occurs in two phases. The two phases include a first phase that includes a first portion of the active braking and a second phase that includes back electromotive force (BEMF) voltage sensing and a second portion of the active braking.

1. A motor control circuit comprising:
 a motor driver circuit to apply a brake current to a coil of an external motor for active braking of the motor; 
 a braking control circuit, coupled to the motor driver circuit and responsive to an externally generated control signal, to control the active braking by the motor drive circuit so that the active braking occurs in two phases; and 
 wherein the two phases include a first phase comprising a first portion of the active braking, and a second phase comprising back electromotive force (BEMF) voltage sensing and a second portion of the active braking. 
 
     
     
       2. The motor control circuit of  claim 1  wherein the braking control circuit comprises circuitry to start the second phase when a first low speed threshold (FLST) is reached during the first phase. 
     
     
       3. The motor control circuit of  claim 2  wherein the FLST is a time-based threshold. 
     
     
       4. The motor control circuit of  claim 1  wherein the braking control circuit comprises circuitry to determine when a second low speed threshold (SLST) is reached during the second phase. 
     
     
       5. The motor control circuit of  claim 4  wherein the SLST is a voltage-based threshold. 
     
     
       6. The motor control circuit of  claim 5  wherein the braking control circuit further comprises a BEMF sensing circuit to obtain a BEMF voltage from the coil and to compare the BEMF voltage to the voltage-based threshold. 
     
     
       7. The motor control circuit of  claim 4  wherein braking control circuit further comprises circuitry to define a maximum time window in which the SLST determination is made. 
     
     
       8. The motor control circuit of  claim 7  wherein the braking control circuit further comprises logic to indicate an end of the second phase when the SLST is reached within the maximum time window. 
     
     
       9. The motor control circuit of  claim 7  wherein the braking control circuit further comprises logic to indicate an end of the second phase when either the SLST is reached within the maximum time window or a maximum timeout has occurred. 
     
     
       10. The motor control circuit of  claim 2  wherein the BEMF sensing comprises intervals of BEMF sensing and the second portion of the active braking comprises intervals of active braking, and wherein the BEMF sensing intervals alternate with the active braking intervals during the second phase according to magnetic transitions of the motor and timing control of the braking control circuit. 
     
     
       11. The motor control circuit of  claim 10  wherein the timing control requires that the second phase be ended when a SLST is reached. 
     
     
       12. The motor control circuit of  claim 1  wherein the braking control circuit comprises a magnetic transition detector to generate an output indicative of magnetic transitions of a magnet in the motor as the motor rotates during the active braking, wherein braking current applied during the active braking comprises braking pulses which begin with each magnetic transition, and wherein the braking control circuit further comprises logic to determine a maximum active braking time (MABT) limit relative to each braking pulse in the first phase and to control the motor drive circuit to discontinue the application of braking pulses in the first phase when the MABT limit is reached. 
     
     
       13. The motor control circuit of  claim 12  wherein the braking control circuit further comprises logic to determine a MABT limit relative to each braking pulse in the second phase and to control the motor drive circuit to discontinue the application of braking pulses in the second phase when the MABT of the second phase is reached. 
     
     
       14. The motor control circuit of  claim 13  wherein the MABT of the first phase differs from the MABT of the second phase. 
     
     
       15. The motor control circuit of  claim 13  wherein the MABT of the second phase is capable of being adapted to behavior of the motor as the second phase progresses. 
     
     
       16. The motor control circuit of  claim 1  wherein the braking control circuit further comprises logic to determine when a maximum timeout measured from a start of the first phase is reached and to end the second phase when the maximum timeout is reached. 
     
     
       17. The motor control circuit of  claim 1  wherein the motor driver circuit comprises circuitry configured as an H-bridge. 
     
     
       18. The motor control circuit of  claim 1  further comprising a magnetic field sensor that includes a Hall sensor and a Hall detector. 
     
     
       19. The motor control circuit of  claim 1  further comprising a magnetic field sensor, and wherein the motor driver circuit, the braking control circuit and the magnetic field sensor are integrated as a semiconductor integrated circuit. 
     
     
       20. A system comprising:
 a system logic controller; and 
 a motor control circuit, coupled to the controller, comprising: 
 a motor drive circuit to apply a brake current to a coil of an external motor for active braking of the motor; 
 a braking control circuit, coupled to the motor drive circuit and responsive to a control signal by the system logic controller, to control the active braking by the motor drive circuit so that the active braking occurs in two phases; and 
 wherein the two phases include a first phase comprising a first portion of the active braking, and a second phase comprising back electromotive force (BEMF) voltage sensing and a second portion of the active braking.